Polyoxometallates
Polyoxometallates and heteropolyacids, both in general and those which can be used to prepare some of the catalysts used in our invention, and their preparation are described in Pope et al., Heteropoly and Isopoly Oxometalates, Springer-Verlag, New York (1983).
Polyoxometallates and heteropolyacids consist of a polyhedral cage structure or framework bearing a negative charge (e.g., [PW.sub.12 O.sub.40 ].sup.-3) which is balanced by cations that are external to the cage. If the cations are protons, then the compound is a heteropolyacid (HPA) (e.g., H.sub.3 [PW.sub.12 O.sub.40 ]). If the cations were not all hydrogen, but either metals such as an alkali metal, potassium, sodium, or lithium, as in K.sub.3 PW.sub.12 O.sub.40, or ammonium, as in (NH.sub.4).sub.3 PW.sub.12 O.sub.40, then it is referred to as a polyoxometallate (POM). In earlier patents, we have used the term "polyoxoanion" to describe compounds in which some or all of the cations are not hydrogen (e.g., K.sub.3 PW.sub.12 O.sub.40 or H(VO)[PW.sub.12 O.sub.40 ]); in the present case, however, these compounds are referred to as polyoxometallates and the term polyoxoanion is reserved for describing the anionic cage-like portion of the compound (e.g., [PW.sub.12 O.sub.40 ].sup.-3).
As described in Pope et al., supra, heteropolyacids and polyoxometallates are cage-like structures with a primary, generally centrally located atom(s) surrounded by a cage framework, which framework contains a plurality of metal atoms, the same or different, bonded to oxygen atoms. The central element of heteropolyacids and polyoxometallates is different from metal atoms of the framework and is sometimes referred to as the "hetero" element or atom; the condensed coordination elements are referred to as the "framework" elements or metals. The framework metal atoms are ordinarily transition metals. As described by Pope et al., supra, the majority of heteropolyacids and polyoxometallates have a centrally located heteroatom ("X") usually bonded in a tetrahedral fashion through four oxygen atoms to the "framework" metals ("M"). The framework metals, in turn, (i) are usually bonded to the central atom in an octahedral fashion through oxygens ("O"), and (ii) are bonded to four other framework metals through oxygen atoms and (iii) have a sixth non-bridging oxygen atom known as the "terminal oxygen" atom. This can be illustrated as shown below: ##STR1##
The principal framework metal, M, is effectively limited to only a handful of metals including molybdenum, tungsten, vanadium, niobium and tantalum. According to Pope et al., supra, this is due to the necessary condition that suitable metals have appropriate cation radius and be good oxygen p.pi.-electron acceptors. Among the successful candidates, molybdenum and tungsten share a common feature; namely, the expansion of valences of their metal cations from four to six. The coincidence of these characteristics allow these metals to form stable heteropolyacids and polyoxometallates.
Conventional heteropolyacids (and polyoxoanions thereof) can be described by the general formula H.sub.e (X.sub.k M.sub.n O.sub.y).sup.-e. In this formula, X, the central atom, is frequently phosphorus. However, other suitable central atoms include Group IIIB-VIB elements, such as antimony, silicon and boron. Further, the subscript k is preferably 1, but can be as high as 5. M is molybdenum, tungsten, or vanadium and n will vary from 5-20. The subscript y is usually about 40, but can be as low as 18 or as high as 62. The notation e is the negative charge on the (X.sub.k M.sub.n O.sub.y) polyoxoanion and will vary from case to case, but e is always the number of protons needed to balance the formula. In a typical such heteropolyacid, k=1, n=12 and y=40 as in H.sub.3 PMo.sub.12 O.sub.40 and the polyoxometallate K.sub.4 PW.sub.11 VO.sub.40.
As described in Pope et al., supra, heteropolyacids are known to exist in a variety of structures including the Keggin, Dawson and Anderson structures. The different structures correspond to the specific geometry of particular heteropolyacid compositions and vary according to the coordination chemistry and atomic radii of the metals present.
Substituted Polyoxometallates
We have earlier disclosed framework-substituted heteropolyacids and polyoxometallates which demonstrated improved activity for the conversion of alkanes to alcohols. Ellis et al., U.S. Pat. No. 4,898,989, issued Feb. 6, 1990. The improvement in catalyst activity was achieved by replacing certain framework atoms M (and the oxygen atoms doubly bonded to them) with zinc or transition metals or combinations thereof. The M atoms thusly replaced are best shown from the following structure: ##STR2## This twelve-cornered polyhedron structure is the metal atom cage-like configuration of a typical Keggin ion heteropolyacid described above. Between any two metal atoms of the framework of the cage is an oxygen atom, not shown, and from each metal atom is also a doubly-bonded oxygen not shown. Each of the metal atoms is bonded through oxygen to the central metal atom, not shown. The structure of polyoxometallates of other kinds (e.g., Dawson ions, Anderson ions) can have different polyhedral structures.
It can be seen from the diagram that eight of the fourteen faces of the above polyhedron are triangular and the other six are four-sided polygons. The M atoms which are replaced, according to our U.S. Pat. No. 4,898,989 patent, supra, are the three metal atoms in a single triangular face, not just any metal atoms as would happen in a random replacement. Another way of characterizing the regioselective, triangular insertion of the substituted metal atoms ("M'"), is that the M' atoms are each joined to each other in the above structural diagram (through oxygen atoms, if the complete structure were shown).
A typical heteropolyacid useful in making the substituted compositions has the formula H.sub.3 PMo.sub.12 O.sub.40. When three Mo=0 units are replaced with, e.g. iron (Fe), the resulting framework substituted heteropolyacid has the formula H.sub.6 PMo.sub.9 Fe.sub.3 O.sub.37. Thus, the general formula of the regioselectively substituted heteropolyacids described above becomes: EQU H.sub.e (X.sub.k M.sub.n M'.sub.m O.sub.y).sup.-e
where k is 1-5, n is 5-19, m is 1-3 and y is 18-61. In this formula, M' comprises zinc or any of the transition metals, namely the Group IIIA-VIII metals of the periodic table. Preferably the transition metal is from Group VIII or the first row of Group IVA-VII, i.e., iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum (Group VIII) or titanium, vanadium, chromium, managanese (IVA-VII, first row). Among the more preferred M' metals are iron, manganese, vanadium and combinations of nickel and iron or other transition metal. The three M' atoms do not have to be the same. However, the three M' must be different than the three M atoms replaced.
Preparation of Polyoxometallates
Heteropolyacids are conventionally prepared by dissolving the desired metal oxides in water, adjusting the pH to approximately 1-2 with acid (e.g. HCl) to provide the necessary H.sup.+ cations, and then evaporating water until the heteropolyacid precipitates. If polyoxometallate is desired, a salt such as KCl is added. The polyoxometallate ordinarily precipitates without need for an evaporation step. The desired proportion of the metal oxides may vary somewhat from the theoretical amount required for the desired product. The existence of the heteropolyacid structure is confirmed by their characteristic NMR and/or IR spectra, which, as explained in Pope et al., supra, are now known for various heteropolyacids.
In our U.S. Pat. No. 4,803,187, issued Feb. 7, 1989, we taught how to prepare heteropolyacids and polyoxometallates with random substitution of framework metals, such as H.sub.7 (PMo.sub.8 V.sub.4 O.sub.40); K.sub.6 (SiMo.sub.11 MnO.sub.39) and K.sub.5 (PW.sub.11 VO.sub.40). The latter, for example, may be prepared by dissolving 45.0 g of 12-tungstophosphoric acid, H.sub.3 (PMo.sub.12 O.sub.40), in 105 ml of water. With stirring the pH is adjusted to about 5.2 with potassium bicarbonate. The mixture is then heated to 70/C and 6.0 g of vanadyl sulfate (VOSO.sub.4) in 15 ml water is added. The solution is cooled and KCl is added to precipitate the K.sub.5 (PW.sub.11 VO.sub.40) product.
The preparation of framework-substituted heteropolyacids or polyoxometallates as described in our U.S. Pat. No. 4,803,187 patent, supra, is adequate for random substitution, but will not provide the regiospecific, trilacunary substitution as described in our U.S. Pat. No. 4,898,989 patent, supra; i.e., replacement of three M in a single, triangular face with three M'. In order to achieve the latter, the following generalized procedures may be employed.
The overall procedure involves the reaction of a trilacunary polyoxoanion with a trimetalacetate, the metals of the latter being those to be inserted into the polyoxoanion. Polyoxoanion ("POA") refers to the anionic portion of the compound which is the negatively charged cage without the external protons or cations. The framework substituted polyoxoanion is then converted to the corresponding heteropolyacid if desired. The trilacunary, Na.sub.9 (PW.sub.9 O.sub.34), for example is prepared by mixing Na.sub.2 WO.sub.4 and H.sub.3 PO.sub.4 in the stoichiometric ratio in water at room temperature for 25 minutes and then slowly acidifyng with 12N HCl to a final pH of 7.1. The Na.sub.9 PW.sub.9 O.sub.34 precipitates and is separated. Other trilacunaries are prepared similarly by known analogous procedures.
It is apparent from the above that the PW.sub.9 O.sub.34 in the trilacunary polyoxoanion represents the removal of three O--W.dbd.O units from the polyoxoanion and not merely W.dbd.O as described for the framework substituted heteropolyacid/polyoxoanion of our prior U.S. Pat. No. 4,803,187. This is merely a matter of satisfying the valences of tungsten (W) in the portion removed. The singly-bonded oxygen in the O--W.dbd.O is reinserted when M' is inserted so that the overall effect is the replacement of a M.dbd.O with M'; thus changing the number of framework oxygen atoms from 40 to 37.
The trimetal acetates have the general formula M.sub.3 O(CH.sub.3 COO).sub.6 (H.sub.2 O).sub.3 where M is a transition metal or zinc and M.sub.3 may be the same or different, e.g., Fe.sub.2 NiO(CH.sub.3 COO).sub.6 (H.sub.2 O).sub.3. They are prepared, e.g., by reaction of appropriate salts. Thus the above diiron-nickel compound is prepared by mixing sodium acetate, iron nitrate, and nickel nitrate in acetic acid/H.sub.2 O at room temperature and separating the precipitate. See Blake et al., J. Chem. Soc. Dalton Trans., p. 2509 (1985); and Uemura et al., J. Chem. Soc. Dalton Trans., p. 2565 (1973).
Once the precursors are prepared, the framework substituted heteropolyacid/polyoxometallate is formed by reacting them together. For example, the trilacunary oxoanion Na.sub.9 (PW.sub.9 O.sub.34) is dissolved in a pH 6, buffered KOAc/HOAc solution (OAc=acetate). Then an equimolar amount of the trimetal acetate, e.g., Fe.sub.2 NiO(OAc).sub.6 (H.sub.2 O).sub.3 dissolved in water is added. After initial mixing, the mixture is stirred for one hour at 50.degree. C. and then cooled to room temperature. KCl is added to precipitate the product K.sub.7 (PW.sub.9 Fe.sub.2 NiO.sub.37). Various preparatory methods are described in Finke et al. J. Amer. Chem. Soc., 108, p. 2947 (1986), F. Ortega, Ph.D. Thesis, Georgetown University (1982), and Domaille et al., Inorg. Chem., 25, 1239-42 (1986).
The polyoxometallate salt can be readily converted to the acid form if desired. This is done by reacting an aqueous solution of the salt, e.g., K.sub.7 PW.sub.9 Fe.sub.2 NiO.sub.37 at 50.degree. C. for 15 minutes with an aqueous solution containing an excess of tetrabutylammonium bromide. Upon refrigeration at 4.degree. C. overnight, the organic salt, (nC.sub.4 N).sub.7 PW.sub.9 Fe.sub.2 NiO.sub.37 crystallizes in 70% yield. The organic salt is filtered off and pyrolyzed at 400.degree. C. for 1 hour. It turns into the black solid H.sub.7 PW.sub.9 Fe.sub.2 NiO.sub.37 as confirmed by IR. The existence of the framework substituted heteropolyacid/polyoxometallate may be confirmed by IR and elemental analysis in known manner.
Regio-disubstituted heteropolyacids and polyoxometallates may be prepared similarly to the procedure described above. A dilacunary species, such as K.sub.8 (SiW.sub.10 O.sub.36), is reacted at pH 3.8 with a dimeric metal formate, such as [Cr.sub.2 (OH)(O.sub.2 CH)](TsO).sub.3, where "TsO" is tosylate anion. The product of this reaction after purification is K.sub.8 (SiCr.sub.2 W.sub.10 O.sub.38), where two W.dbd.O units have been replaced by two Cr.sup.III atoms.
Catalytic Oxidation
As described in Pope et al., supra, heteropolyacids and polyoxometallates have found a variety of applications. In the area of catalysis, they have been used in connection with the oxidation of propylene and isobutylene to acrylic and methacrylic acids, oxidation of aromatic hydrocarbons; olefin polymerization; olefin epoxidation; and hydrodesulfurization processes.
The use of heteropolyacids and polyoxometallates for the catalytic air oxidation of alkanes to alcohols, such as butane to butanol, is also known. See, for example, M. Ai, "Partial Oxidation of n-Butane with Heteropoly Compound-based Catalysts", Proceedings of the 18th International Congress on Catalysis, Berlin, 1984, Verlag Chemie, Vol. 5, page 475. In addition, we have previously disclosed the use of heteropolyacids and polyoxometallates under mild reaction conditions for the liquid phase oxidation of alkanes. See, Lyons et al., U.S. Pat. No. 4,803,187, supra. That patent is incorporated by reference herein.
Further, we have previously disclosed modified heteropolyacids and polyoxometallates, methods of preparation, and methods of use for oxidation of alkanes to alcohols. See, Lyons et al., U.S. Pat. No. 4,859,798, issued Aug. 22, 1989; Ellis et al., U.S. Pat. No. 4,898,989, supra; Lyons et al., U.S. Pat. No. 4,916,101, issued Apr. 10, 1990; Ellis et al., U.S. Pat. No. 5,091,354, issued Feb. 25, 1992; and Shaikh et al., U.S. Pat. No. 5,334,780, issued Aug. 2, 1994; each of which is incorporated herein by reference.
We have previously found that substitution of Group VIII and other transition metals as framework elements in a heteropolyacid or polyoxometallate catalyst enhances catalytic oxidation activity for the oxidation of alkanes to alcohols. See, Ellis et al., U.S. Pat. No. 4,898,989, supra; and Ellis et al., U.S. Pat. No. 5,091,354, supra.
Framework-substituted heteropolyacids similar to those described by Ellis et al. and Lyons et al., supra, were subsequently disclosed as catalysts for oxidation of aldehydes, cyclohexene and cyclohexane, and for hydrogen peroxide decomposition. N. Mizuno et al., "Synthesis of [PW.sub.9 O.sub.37 {Fe.sub.3-x Ni.sub.x (OAc.sub.3 }].sup.(9+x)- (x=predominantly 1) and Oxidation Catalysis by the Catalyst Precursors", J. Mol. Cat., 88, L125-31 (1994), and Wu et al., "Catalytic Behavior of Metal Ions Located at Different Sites of Heteropoly Compounds", Catalysis Letters, 23, 195-205 (1994).
Production of Carboxylic Acids
Non-framework substituted polyoxometallates and heteropolyacids are known in the art as catalysts for oxidation of isobutane to methacrylic acid and methacrolein. W. Ueda et al., "Catalytic Oxidation of Isobutane to Methacrylic Acid with Molecular Oxygen over Activated Pyridinium 12-Molybdophosphate", Cat. Lett., 261-265 (1997); N. Mizuno et al., "Catalytic Performance of Cs.sub.2.5 Fe.sub.0.08 H.sub.1.26 PVMo12O.sub.40 for Direct Oxidation of Lower Alkanes", J. Mol. Catal., A, 114, 309-317 (1996); F. Trifiro, "Reactivity of Keggin-type Heteropolycompounds in the Oxidation of Isobutane to Methacrolein and Methacrylic Acid: Reaction Mechanism", J. Mol. Catal., A, 114, 343-359 (1996); N. Mizuno et al., "Direct Oxidation of Isobutane into Methacrylic Acid and Methacrolein over Cs.sub.2.5 Ni.sub.0.08 -substituted H.sub.3 PMo.sub.12 O.sub.40 ", J. Chem. Soc., Chem. Commun., 1411-1412 (1994); S. Yamamatsu et al., "Process for Producing Methacrylic Acid and Methacrolein", European Patent Specification Publication No. 0 425 666 B1, Application No. 89905775.6 filed May 22, 1989, Date of publication of patent specification Apr. 13, 1994; S. Yamamatsu et al., "Method for the Fabrication of Methacrylic Acid and/or Methacrolein", Japanese Patent Application Public Disclosure No. H2-42034, Feb. 13, 1990; S. Yamamatsu et al., U.S. Pat. No. 5,191,116, issued Mar. 2, 1993; K. Nagai et al., Process for producing methacrylic acid and methacrolein by catalytic oxidation of isobutane", European Patent Application Publication No. 0 418 657 A2, Application No. 90117103.3, filed Sep. 5, 1990 by Sumitomo Chem.Ind.KK (published Mar. 27, 1991).
T. Jinbo et al., "Method for the Manufacture of Acroleic Acid or Acrylic Acid, and Catalysts Used Therein", Japanese Patent Application Public Disclosure No. H6-218286, Aug. 9, 1994, discloses the conversion of propane to acrolein and/or acrylic acid catalyzed by extra-framework metal substituted heteropolyacids; i.e., cation-exchanged heteropolyacids. A single framework mono-substituted heteropolyacid, H.sub.4 PMo.sub.11 VO.sub.40, showed moderate selectivity for acrylic acid and poor conversion rate.
A heteropolyacid containing ten molybdenum atoms and two vanadium atoms randomly substituted in its framework has been disclosed as catalyzing the oxidation of n-butane to maleic anhydride, acrylic acid and acetic acid. M. Ai, "Partial Oxidation of n-Butane with Heteropoly Compound-based Catalysts", Labo. Resources Utiliz., Tokyo Inst. Tech., Yokohama, Japan, 8th International Congress on Catalysis, Volume V: Cluster-derived catalysts, Active phase support interactions, Catalysts for synthesis of Chemicals, Verlag Chemie, Berlin, pages V475-V486 (1984).
G. Centi et al., "Selective Oxidation of Light Alkanes: Comparison between Vanadyl Pyrophosphate and V-Molybdophosphoric Acid", Catal.Sci.Technol., Proc. Tokyo Conf., 1st Meeting, 1990, 225-30, 227, disclose that the randomly framework-substituted H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 has been found to be more active than (VO).sub.2 P.sub.2 O.sub.7 for catalyzing oxidation of propane to acrylic acid. However, the heteropolyacid was inactivated within 1.5 hours. The reported results may suggest that the H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 composition was not functioning as a catalyst, but was rather functioning as a stoichiometric reagent.
Partially exchanged Cs-salts of heteropolyacids have been found to be more active than pure heteropolyacids for catalyzing oxidation of lower alkanes. N. Mizuno et al., "Catalytic Performance of Cs.sub.2.5 Fe.sub.0.08 H.sub.1.26 PVMo.sub.11 O.sub.40 for Direct Oxidation of Lower Alkanes", J. Mol. Catal., A, 114, 309-317 (1996).
When a combination of the unsubstituted heteropolyacid, H.sub.3 PMo.sub.12 O.sub.40, and V.sub.2 O.sub.5 --P.sub.2 O.sub.5 is used to catalyze oxidation of propane to acrylic acid, this unsubstituted heteropolyacid is disclosed as enhancing the formation of acetic acid byproduct. M. Ai, "Oxidation of Propane to Acrylic Acid", Catalysis Today, 13 (4), 679-684 (Eng.) (1992).
N. Mizuno et al., Applied Catalysis A: General, 128, L165-L170 (1995), reported that Fe.sup.+3 or Ni.sup.+2 exchange for H.sup.+ ; and V.sup.+5 mono-substitution for Mo.sup.+6 in CS.sub.2.5 H.sub.0.5 PMo.sub.12 O.sub.40 enhanced the catalytic activity for direct oxidation of propane to acrylic acid. Of the catalysts tested, Cs.sub.2.5 Fe.sub.0.08 H.sub.0.5 PMo.sub.11 VO.sub.40 gave the highest yield of acrylic acid.
Ueda et al., Chemistry Letters, 541, 2 (1995), reported that propane was catalytically oxidized to acrylic acid and acetic acid with molecular oxygen over unsubstituted heteropolymolybdophosphoric acids which were treated with pyridine.
Cavani et al., Catalysis Letters, 32 215-226 (1995), reported that the addition of iron salts led to a substantial increase in the activity of unsubstituted 12-molybdophosphoric acid for the oxidation of isobutane to methacrylic acid.
The references cited above primarily employed non-framework substituted heteropolyacids as catalysts in manufacture of unsaturated carboxylic acids, for example acrylic acid and methacrylic acid, from alkanes, for example propane and isobutane. As noted, there has also been some use disclosed of heteropolyacids and polyoxometallates with random vanadium substitution of one or two framework metals. The yields and selectivities from the use of vanadium pyrophosphates and/or heteropolyacids, including random, mono- and divanadium-substituted heteropolyacids, described in the cited references was generally below the level required for a practical process. There has been no prior disclosure or use of site-specific, regioselective di-, tri- or multi-substituted heteropolyacids or polyoxometallates for the conversion of alkanes to unsaturated carboxylic acids.
Given the value and industrial importance of acrylic acid and methacrylic acid, it has been recognized that the one-step conversion of alkanes to unsaturated carboxylic acids would be a useful process with important commercial applications, provided that sufficient yield can be obtained. To date, no efficient catalysts have been developed for the commercial production of acrylic acid from propane or methacrylic acid from isobutane. As a result, acrylic acid is manufactured from propylene, a raw material which is over three times more expensive than propane.
The process of the present invention provides such a one-step process for the conversion of alkane to carboxylic acid. The advantages of the process according to the invention are that the higher catalytic activities of the catalysts used in the process allow the process to be carried out at lower temperatures than those used in the prior art, and to obtain higher reaction rates, yields and selectivities than those obtained in the prior art. These advantages make the process more attractive than the prior art processes for practical use and potential commercial interest.
It has been found that the yield and/or selectivity for unsaturated carboxylic acids and nitriles in the partial oxidation of alkanes catalyzed by heteropolyacids and/or polyoxometallates, as such or in combination with vanadium pentoxides, may be increased by substituting oxidation-active metals, for example iron, in Keggin and Dawson structures for metals, for example molybdenum, in the Keggin or Dawson structure to obtain superior catalysts for the direct oxidation of alkanes to unsaturated carboxylic acids, for example, acrylic or methacrylic acids, or for the oxidation of alkanes in the presence of a nitrogen compound to form unsaturated nitrites, for example acrylonitrile or methacrylonitrile.